Power Plant

ABSTRACT

A process and a power plant comprising a combustor with a flue gas outlet and one or more water selective separators downstream the flue gas outlet to separate water from the flue gas. Optionally, a water return line returns separated water from the water selective separator to a water inlet of a CO shift cell with an outlet feeding H 2  enriched syngas to the combustor.

The invention relates to a power plant comprising a syngas combustor, such as for example a combustor in an integrated gasification combined cycle (IGCC) plant. Such IGCC plants typically include an upstream gasifier to produce syngas by partial combustion of a hydrocarbon feed, such as pulverized coal, and a downstream combustor to combust the syngas produced in the upstream gasifier.

Syngas is a mixture of carbon monoxide and hydrogen. Yields of generated energy are higher with syngas having higher hydrogen content. The hydrogen content in the syngas can be increased by the water-gas shift reaction. With this reaction carbon monoxide reacts with water to form carbon dioxide and hydrogen:

CO(g)+H₂O(1)→CO₂(g)+H₂(g)

Heat can be extracted from the flue gas discharged from the combustor to be used for generating power, typically via a steam cycle.

The flue gas includes water vapour. The water content in the flue gas is mainly produced by the combustion of hydrogen in the combustor. Water is also introduced by the syngas fuel and by air supplied to the combustor. A further source of the flue gas water content is water used for washing the flue gas to remove pollutants, such as SO_(x), NO_(x) and fly ash.

Due to the high water vapour content of the flue gas, the temperature of the flue gas should be maintained well above the dew point to prevent corrosion of plant equipment. As a consequence, part of the heat in the flue gas cannot be recovered and used for power generation.

It is an object of the invention to reduce consumption of energy and to improve the overall efficiency of the combustion process.

The object of the invention is achieved with a power plant, comprising a combustor with a flue gas outlet and one or more water selective separators downstream of the flue gas outlet to separate water from the flue gas.

Since the water content of the flue gas is substantially reduced, the dew point is lowered and more heat can be recovered from the flue gas. This contributes to a substantial improvement of the overall efficiency of the power generating process.

It has been found that about 70% of the water content of the flue gas can be recovered. This water has a high degree of purity and can for example be reused as make-up water for a steam cycle. This reduces the need to prepare fresh make-up water, which results in substantial energy savings, particularly if the make-up water is prepared from sea water, e.g., by desalination or by reverse osmosis using membrane filtration.

The recovered water can also be used as cooling water in any part of the process or for any other suitable purpose.

In a specific embodiment, the plant may further comprise a CO shift cell with a syngas inlet, a water inlet and an outlet operatively connected to the inlet of the combustor for the transport of H₂ enriched syngas to the combustor, wherein a water return line is configured to return at least a part of the separated water from the separator to the water inlet of the CO shift cell. In the CO shift cell, the hydrogen content of the syngas is substantially increased by the water-gas shift reaction. CO₂ formed with this reaction is separated and discharged, for instance by scrubbing using amine solvents. This way, the hydrogen content of the enriched syngas fed to the combustor is substantially increased without increasing water consumption in the process as a whole.

The water selective separator can for instance be a filter membrane. Suitable membranes include hydrophilic stainless steel membranes, such as tubular stainless steel membranes of Hyflux, and hydrophilic ceramic membranes, such as ceramic membranes from CEPAration in Helmond, the Netherlands. The ceramic membranes can for instance be made of a-alumina.

The membranes can for instance include ultrafiltration membranes, with average pore sizes of 20-50 nm or microfiltration (MF) membranes with average pore sizes of about 200-1000 nm.

The water can for example be removed from the flue gas by permeation through the membrane, driven by a mild underpressure, e.g., of about 0.05-0.25 bar, and subsequent condensation. Alternatively, or additionally, water can be removed from the flue gas using other types of separators, such as a condenser or desiccants. Optionally, the plant may comprise more than one water selective separator, e.g., in series or in parallel arrangement. The separators can be of the same type or of different types.

The plant may include one or more gasifiers to produce syngas by partial combustion of a hydrocarbon feed, such as pulverized coal, biomass, oil or mixtures thereof. Such a gasifier typically comprises a number of burners supplying an oxygen containing gas, such as air, to the gasifier and an outlet for the discharge of syngas. The syngas can subsequently be treated in a CO shift cell to increase the hydrogen content.

The power plant can be used for a process for generating power by combustion of syngas in the combustor. Water is separated from flue gas from the combustor and can be reused, e.g., by feeding it to the syngas to increase its hydrogen content before the syngas is fed to the combustor. The water can also be reused as cooling water or as make-up water for a steam cycle. The invention will be elucidated with reference to the accompanying drawing.

FIG. 1: shows a diagrammatic presentation of an integrated gasification combined cycle (IGCC) plant.

FIG. 1 shows a flow diagram schematically representing a plant 1 with a feed line 2 for the supply of a hydrocarbon feed, such as pulverized coal. The feed line 2 leads to a gasifier 3, where the hydrocarbon feed is partially combusted to produce syngas. Oxygen containing gas is injected into the gasifier 3 via burners (not shown) to achieve partial combustion of the hydrocarbon feed for the production of syngas. Raw syngas is discharged and washed in a cleaning unit 4 to remove pollutants, such as fly ash. The cleaned syngas mainly comprises hydrogen gas H₂ and CO.

The cleaned syngas is subsequently passed to a CO shift cell 5, where water vapour is mixed into the syngas flow to initiate a water-gas shift reaction. Water reacts with CO to form CO₂ and H₂. The resulting CO₂ is separated and discharged via a discharge line 6. An enriched syngas is obtained with a high H₂ content. The water-gas shift process can for example be performed in two stages: a first stage comprising a high-temperature shift at about 350° C. followed by a second stage at about 190-210° C. Catalysts can be used, such as iron oxide promoted with chromium oxide for the first stage and copper on a mixed support composed of zinc oxide and aluminum oxide for the second stage.

The resulting enriched syngas is then fed to a combustor 7. An oxygen containing gas, such as air, is fed into the combustor 7 via burners. The H₂ content of the syngas reacts with the injected O₂ to form water.

The combustor 7 will typically be integrated in a steam cycle for using the generated heat to generate power by a steam turbine. Hot flue gas, typically having a temperature of about 350° C. is discharged via a flue gas outlet 8 to a filter unit 9 comprising a water selective hydrophilic membrane 10, such as a hydrophilic ceramic membrane. Water permeates through the membrane 10 and is subsequently condensed. In this stage about 70 wt % of the water content in the flue gas can be recollected.

Part of the separated water is subsequently returned via a return line 11 to the CO shift cell 5 and used in the water-gas shift process.

The filtered and dehydrated flue gas is passed from the filter unit 8 to a downstream heat recovery unit 12 where heat is recovered from the hot flue gases to maximize energy efficiency. In this stage the flue gasses are cooled to a temperature above the dew point to prevent corrosion of the used equipment. Since the flue gases are dehydrated, the dew point is substantially lowered, so more heat can be recollected from the flue gases. As a result, the overall process efficiency is substantially improved.

Part of the water separated by the filter unit 8 is transported via a line 13, e.g., for reuse as make-up water in a steam cycle or as cooling water.

The membrane filter 10 can for instance be a hollow fiber membrane positioned in a flow path of flue gas discharged from a combustor, preferably as much downstream as possible.

The hollow fiber membrane may for instance comprise an array of tubular modules. Hot flue gas passes the modules, which extract water from the flue gas. The outlets of the modules can connect to a vacuum chamber where the recovered water is collected and condensed. 

1. A power plant comprising a combustor with a flue gas outlet and one or more water selective separators downstream of the flue gas outlet to separate water from the flue gas.
 2. A power plant according to claim 1 comprising a heat recovery unit downstream of the one or more water selective separators for extracting heat from the flue gas.
 3. A power plant according to claim 1 further comprising a CO shift cell with a syngas inlet, a water inlet and an outlet operatively connected to a syngas inlet of the combustor for the transport of H₂ enriched syngas to the combustor, wherein a water return line is configured to return separated water from the one or more water selective separators to the water inlet of the CO shift cell.
 4. A power plant according to claim 1, wherein the one or more water selective separators comprise a hydrophilic ceramic membrane.
 5. A power plant according to claim 1 wherein the one or more water selective separators comprise a hydrophilic stainless steel membrane.
 6. A power plant according to claim 1 wherein the one or more water selective separators include an ultrafiltration membrane.
 7. A power plant according to wherein the one or more water selective separators include an microfiltration membrane.
 8. A process for generating power by combustion of a syngas feed in a combustor, wherein water is separated from flue gas discharged from the combustor.
 9. A process according to claim 8, wherein at least a part of the separated water is returned to the syngas feed to increase its hydrogen content by a water-gas shift reaction before the syngas feed is fed to the combustor.
 10. A process according to claim 8, wherein at least a part of the separated water is fed to a steam cycle as make-up water.
 11. A process according to claim 1, wherein at least a part of the water is separated from the flue gas by permeation through one or more membranes followed by condensation. 